Refactor NTA to use RCU instead of rwlock

Refactor NTA to use RCU instead of rwlock

Replace the ntatable rwlock with RCU read-side critical sections.
The QP multi trie already provides its own concurrency control for
reads and writes, making the rwlock redundant. NTA fields like
expiry are only accessed from the NTA's own event loop thread, so
no additional synchronization is needed.

The table shutdown is now deferred via call_rcu to ensure all
read-side critical sections have completed before iterating and
shutting down individual NTAs.